The present invention generally relates to films useful as tape backings, and more particularly to multilayer laminate biaxially-oriented backings comprising a propylene-containing primary layer and at least one polyolefin-containing secondary layer and further including a surface finishing layer, such as a surface matte layer.
Commercially available pressure sensitive adhesive tapes are usually provided in a roll form and can be provided on a tape dispenser (see e.g. U.S. Pat. Nos. 4,451,533 and 4,908,278). Typically, commercially available dispensers have either a metal or plastic serrated cutting blade. In the case where no dispenser is provided, it is desirable that the tape can be torn by hand.
xe2x80x9cSeverabilityxe2x80x9d of adhesive tape is defined as the ability to cut or sever a length of tape by pulling the tape over the teeth on the serrated cutting edge of a tape dispenser with a desired amount of energy or work. Severability is also referred to as xe2x80x9cdispensability.xe2x80x9d It is desired that the severed tape does not chip, sliver, fracture or break in an unpredictable manner (see U.S. Pat. Nos. 4,451,533 and 4,908,278). Such severability is desirable to produce a cleanly serrated cut edge on the severed tape strip.
Adhesive tapes are produced from many different films. Films that tend to be too tough and stretch significantly before breaking while under load, render such films extremely difficult to sever, particularly on a plastic bladed dispenser. Tape dispensers with plastic cutting blades are commonly used for hand-held units such as a Catalog No. 104 3M Magic(trademark) tape dispenser, available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. However, dispensers with plastic cutting blades are typically not sharp and durable enough to sever oriented polypropylene tapes. For this reason, typical dispensers for biaxially oriented polypropylene tape usually are equipped with sharp metal blades. Such metal blade dispensers are more costly and difficult to manufacture than dispensers with plastic blades.
Biaxially oriented isotactic polypropylene films are well known for their toughness, low cost, good color, moisture and slivering resistance, and clarity and have long been used as adhesive tape backings (see U.S. Pat. Nos. 3,241,662 and 3,324,218). Although, as stated above such films are more difficult to dispense, particularly using a plastic dispenser. In addition, such films are considered very difficult to tear by hand and are typically slit with roughened or flattened rotary blades in order to provide sufficient edge defects to allow some degree of finger tearability. This process results in non-uniform, unsightly edges and much reduced tape strength.
There have been several attempts to improve polyolefin films to produce severable and finger tearable adhesive tape backing films.
For example, JP 53034834 describes a mixture of polypropylene with low molecular weight polyolefins in order to produce a more brittle, finger tearable film backing. U.S. Pat. No. 3,887,745 describes a 2-layer polypropylene tape in which a thick secondary layer is uniaxially oriented in the transverse (or perpendicular) direction to facilitate straight line tearing in the tape crosswise direction. Similarly, U.S. Pat. No. 3,952,073 and U.S. Pat. No. 4,045,515 describe tape backings comprising blends of isotactic polypropylene with random propylene-ethylene copolymers, which are sequentially oriented to give a film having greater orientation in the transverse than in the machine direction in order to produce film having good finger tearability across the film.
U.S. Pat. No. 4,410,582 describes a sequentially biaxially oriented multilayer film consisting of a low molecular weight polyolefin primary layer having a melting point lower than higher crystallinity secondary layers in order to produce a finger tearable film backing. U.S. Pat. No. 4,137,362 describes adhesive tape backings produced by the sequential biaxial orientation of single layer blends of isotactic polypropylene with other polyolefins. U.S. Pat. No. 4,393,115 and U.S. Pat. No. 4,414,261 and JP Application 11-1998835 describe sequentially biaxially oriented multilayer films which include polypropylene-ethylene block copolymer or hydrocarbon resins in blends or layers, which are intended to promote finger tearability by increasing the film""s brittle behavior.
Similarly, U.S. Pat. No. 4,447,485 and U.S. Pat. No. 4,513,028 describe stretched film having a base sheet comprising polypropylene blended with polymethylpentene, the latter present to produce a more brittle construction. JP Application 6-305014 describes a single composition, single sheet film comprising a polypropylene-ethylene copolymer which develops a secondary/primary structure due to thermal processing conditions using a sequential biaxial orientation process.
Pressure sensitive adhesive tapes with a matte surface opposite the adhesive coated surface are commonly sold for use in offices, retail shops and schools. These tapes are typically used in conjunction with paper, for mending tears, posting memos, and attaching documents together, and as such, it is preferred that the pressure sensitive tape remain invisible in its final use, yet allow for any printing underneath to be viewed and photocopied clearly and cleanly. Additionally, it is preferred that the matte surfaces of these tapes can be written on with a variety of writing instruments, such as ball-point pens, pencils and permanent markers. Cellulose acetate film with an embossed matte finish has been predominantly used as the substrate for matte adhesive tape due to its excellent optical properties and write-on characteristics.
Biaxially oriented polypropylene (BOPP) films are widely used as film substrates for adhesive tapes due to their excellent strength, moisture resistance and low cost. Generally, BOPP films cannot be imparted with a matte and/or writable surface simply by embossing the surface of the film. Typically, such methods involve the provision of an additional layer atop the base polypropylene layer of the BOPP film, wherein some of the desired properties of a matte or writable surface are provided by the additional layer.
Such known additional layers have frequently been produced by combining two or more polymers in the form of a blend. At least one of the polymers in such a blend is usually a polyolefin, as this provides adhesion to the polypropylene main layer in the BOPP film. Polyolefin-based components in such blends have included polypropylene, co- or terpolymers based on propylene, polyethylenes such as high density polyethylene (HDPE), polypropylene/polyethylene block copolymers, ethylene based co- or terpolymers modified with polar groups, polymers containing butene-1 monomer, and methylepentene polymer. Non-polyolefin components in such blends have included thermoplastics such as polystyrene, polyamides, polyalkyl metbacrylates, and polyesters. Additionally, some of the known additional layer compositions have included inorganic fillers, such as silica, calcium carbonate, or clay. Examples of such known additional layers can be found in U.S. Pat. Nos. 5,501,905, 5,474,820, 5,425,990, 5,366,796, 5,364,704, 4,960,637, 4,513,028, 4,447,485. EP 03382431, and JP 76032668B.
The problem remains, however, that none of the known methods satisfactorily provides a matte surface that combines all of the desirable attributes of a matte appearance, is writable with pencil, ball-point pen, and permanent ink markers, does not xe2x80x9cghostxe2x80x9d on photocopying, and is easy to produce. In particular, there is a need for a matte layer composition that provides all the aforementioned properties but does not require slow cooling of the extruded film sheet to optimize the matte appearance, and does not limit the temperature range useful for stretching the film.
However, none of the alternatives set forth above provides an adhesive tape backing combining all the desirable attributes of strength, easy dispensability, easy finger tear, good clarity, good mechanical strength, and cost effectiveness.
Briefly, in one aspect of the present invention, a multi-layered biaxially oriented film is provided comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a propylene-containing polymer having a first melting point and the secondary layer comprises a propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched. Preferably, this stretched film exhibits a crystalline orientation as determined by wide angle X-ray scattering (WAXS) measurements from the monoclinic (110) crystalline planes that is isotropic or exhibits at most a single specific azimuthal scan maximum, said maximum being positioned at an angle of up to xc2x175xc2x0 from the machine direction (MD). The single azimuthal scan maximum in addition possesses an angular full width at half peak height (FWHM) between about 40xc2x0 to 75xc2x0.
In a further embodiment, each layer may be comprised of homopolymers, copolymers, blends and the like. In such an embodiment, the present invention provides a multilayer film wherein (a) the primary layer comprises at least 50% of the total multilayer thickness (excluding an adhesive composition layer) and contains at least 50% of a first polymer component, denoted as the major component of the primary layer, and (b) the secondary layer contains at least 50% of a second polymer component, denoted as the major component of the secondary layer(s) such that the differential scanning calorimetry melting point (MPdsc) of the second polymer component is greater than the MPdsc of the first polymer component.
Optional polymer components may also be included in the primary layer. Furthermore, the optional polymer component of the primary layer may have a MPdsc about the same or different than the first polymer component, provided that this optional polymer component or components are present in levels so that desirable film attributes are maintained. The optional polymer component(s) may be any suitable polymer. Generally, to obtain desirable low levels of haze, optional polymer components include polyolefin-containing polymers, such as homopolymers, copolymers, terpolymers and the like, more preferably homopolymers or copolymers of polypropylene.
Optional intermediate layers may be used, and can be composed predominantly of either the primary layer or secondary layer polymer, or may be a blend or mixture of the two and further may contain additional components so as to optimize processability and film properties. In order to aid processability, it is preferred that the melt flow rate (MFR) of the various layers has similar values.
Advantageously, the present invention provides severable and finger tearable adhesive polyolefin-based tapes exhibiting a combination of characteristics, particularly low puncture resistance, low finger tear resistance, low haze, low tensile strength in the MD and a refractive index difference (xcex94n).
The film is biaxially stretched so as to provide a unique film having a combination of the many preferred and useful characteristics. The films of the present invention do not necessarily exhibit all of the characteristics in a single embodiment, but rather the film is such that the characteristics can be tailored for specific purposes. Useful characteristics include but are not limited to:
1. A crystalline orientation determined by wide angle X-ray scattering (WAXS) measurements from the monoclinic (110) crystalline planes that is isotropic or exhibits at most a single specific azimuthal scan maximum.
2. The maximum being positioned at an angle within about xc2x175xc2x0, preferably about xc2x145xc2x0, and more preferably about xc2x125xc2x0 with respect to reference MD.
3. The breadth of the single azimuthal scan maximum (the angular full width at half peak maximum) of from about 40xc2x0 to about 75xc2x0.
4. A melting point difference between the major components of the primary and secondary layers of at least about 2xc2x0 C. as measured using differential scanning calorimetry (DSC) as described herein, preferably at least about 5xc2x0 C. and more preferably at least about 8xc2x0 C., wherein the melting point of the primary layer is lower than the melting point of the secondary layer.
5. A refractive index difference, calculated by subtracting the refractive index measured in the TD from that in the MD, greater than about xe2x88x923.0xc3x971031 3, preferably greater than or equal to zero.
6. Puncture energy up to 20 J/cm2, preferably up to 15 J/cm2, and more preferably up to 10 J/cm2 when 2.54 cm wide samples are tested by the method set out below, or up to 90 J/cm2 when 1.27 cm wide samples are tested by the method set out below.
7. Puncture elongation up to about 1.3 cm, preferably up to about 1.0 cm when 2.54 cm wide samples are tested by the method set out below, or up to 2.0 cm when 1.27 cm wide samples are tested by the method set out below.
8. Tensile strength up to 140 MPa in the MD.
9. A transmission haze of the primary and secondary layers according to ASTM D 1003-97 of less than about 4.0%, preferably less than about 2.0%, and more preferably less than about 1.0%.
In one preferred embodiment, the present invention provides a film comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a major component of a propylene-containing polymer having a first melting point and the secondary layer comprises a major component propylene-containg polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched, wherein the stretched film exhibits a crystalline orientation as determined by wide angle x-ray scattering (WAXS) measurements from the monoclinic (110) crystalline planes that exhibits no distinct peak and at most a single specific azimuthal scan maximum, said maximum being positioned at an angle of up to xc2x175xc2x0 from the machine direction (MD). The single azimuthal scan maximum in addition possesses an angular full width at half peak height (FWHM) between about 40xc2x0 to 75xc2x0.
In another embodiment, the present invention provides a film comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a major component of a propylene-containing polymer having a first melting point and the secondary layer comprises a major component propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched, wherein the stretched film exhibits a refractive index difference, calculated by subtracting the refractive index measured in the TD from that in the MD, greater than about xe2x88x923.0xc3x9710xe2x88x923.
In another embodiment, the present invention provides a film comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a major component of a propylene-containing polymer having a first melting point and the secondary layer comprises a major component propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched, wherein the stretched film exhibits a refractive index in the machine direction (MD) about the same or greater than that in the transverse direction (TD); put another way, the refractive index difference, calculated by subtracting the refractive index measured in the TD from that in the MD, is greater than or equal to zero. If this difference is about zero then the film orientation is said to be isotropic.
In yet another embodiment, the present invention provides a film comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a major component of a propylene-containing polymer having a first melting point and the secondary layer comprises a major component propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched, wherein the stretched film exhibits a crystalline orientation as determined by wide angle x-ray scattering (WAXS) measurements from the monoclinic (110) crystalline planes that is isotropic or exhibits at most a single specific azimuthal scan maximum, said maximum being positioned at an angle of up to xc2x175xc2x0 from the machine direction (MD), and a refractive index difference, calculated by subtracting the refractive index measured in the TD from that in the MD, greater than about xe2x88x923.0xc3x9710xe2x88x923.
In yet another embodiment, the present invention provides a film comprising at least one primary layer and at least one secondary layer, wherein the primary layer comprises a major component of a propylene-containing polymer having a first melting point and the secondary layer comprises a major component propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point and the multi-layered construction is biaxially stretched, wherein the stretched film exhibits a crystalline orientation as determined by wide angle x-ray scattering (WAXS) measurements from the monoclinic (110) crystalline planes that is isotropic or exhibits at most a single specific azimuthal scan maximum, said maximum being positioned at an angle of up to xc2x175xc2x0 from the machine direction (MD), and a refractive index difference, calculated by subtracting the refractive index measured in the TD from that in the MD, greater than or equal to zero.
To provide for different surface finishes, at least one surface finishing layer may be included and is not limited to compositions providing writable or matte surfaces, release surfaces, and the like.
In an alternative embodiment, a matte surface multi-layered film is provided comprising at least one primary layer, at least one secondary layer and at least one surface finishing layer, wherein the primary layer comprises a propylene-containing polymer having a first melting point, the secondary layer comprises a propylene-containing polymer having a second melting point, such that the second melting point is greater than the first melting point, and the surface finishing layer comprises a blend or mixture of incompatible polyolefins that produce a phase-separated system, and an inorganic or organic filler particle such that the surface finishing layer has a matte appearance and the multi-layered film is biaxially stretched.
Advantageously, the matte surface multi-layered films of the present invention are significantly free of voids or cracks, with haze of 70-90% and total percent light transmittance of greater than 90% as measured by ASTM D1003 (xe2x80x9cHaze and Luminous Transmittance of Transparent Plasticsxe2x80x9d), gloss of less than or equal to 10% as measured at an incident angle of 60xc2x0 by ASTM D2457-97 (xe2x80x9cSpecular Gloss of Plastic Films and Solid Plasticsxe2x80x9d). Further, the surface finishing layer can be a writable layer using the likes of pencil, ballpoint pen, or permanent marker.
Advantageously, films of the present invention may be hand tearable. It has been found that when a film has puncture energy of up to 10 J/cm2 (as tested on a 2.54 cm wide sample) and a tensile strength of up to 100 MPa, the film is moderately easy to tear by hand. When the film has a puncture energy of up to 5 J/cm2 (as tested on a 2.54 cm wide sample) and a tensile strength of up to 50 MPa, the film is easier to tear by hand.
Additionally, all of the embodiments may be constructed in such a way as to include additional primary, secondary and surface finishing layers, such as a construction comprising a first primary layer, a secondary layer and a second primary layer. Furthermore, intermediate layers may also be used, and include tie layers, primer layers, barrier layers and the like. Protective layers may also be added, without limiting the scope of the present invention. The multi-layered films provided for above can be coated with an adhesive layer to make hand tearable, dispensible, writable, matte and any combinations thereof.
The present invention provides films described above, tape backings made from such films, tapes including the backings, and methods of making the films, backings, and tapes.
As used in this application:
xe2x80x9cbiaxially stretched,xe2x80x9d when used herein to describe a film, means the film has been stretched in two different directions, a first direction and a second direction, in the plane of the film. Typically, but not always, the two directions are substantially perpendicular and are in the longitudinal or machine direction (xe2x80x9cMDxe2x80x9d) of the film (the direction in which the film is produced on a film-making machine) and the transverse direction (xe2x80x9cTDxe2x80x9d) of the film (the direction perpendicular to the MD of the film). The MD is sometimes referred to as the Longitudinal Direction (xe2x80x9cLDxe2x80x9d). Biaxially stretched films may be sequentially stretched, simultaneously stretched, or stretched by some combination of simultaneous and sequential stretching. Further, such stretching can result in films that are balanced or unbalanced. Films having an anisotropic molecular orientation may exhibit anisotropy aligned parallel to any major film axis, so long as the desirable property attributes described herein are met.
xe2x80x9cbirefringencexe2x80x9d when used herein to describe a film means the film has different refractive index values as measured along two perpendicular axes within the plane of the film, that is, the plane normal to the incident linearly polarized light used to measure the refractive index. This so-called xe2x80x9cin-planexe2x80x9d birefringence is the difference in the refractive index parallel to a defined direction and that measured perpendicular to the first direction (Encyclopedia of Polymer Science and Engineering, 2cd ed., v. 14, pp. 552-562, Wiley-Interscience, NY (1987). In the present case, the birefringence (denoted (xcex94nMxe2x88x92T)) is defined as the value of the refractive index measured parallel to the machine or longitudinal direction (denoted nM) minus that measured perpendicular to the machine direction (denoted nT):
(xcex94nMxe2x88x92T)=nMxe2x88x92nT. 
where the measured value of the refractive indices is accurate to the fourth digit (Encyclopedia of Polymer Science and Engineering, 2cd ed., v. 10, p 605-608, Wiley-Interscience, NY (1987)).
xe2x80x9csimultaneously biaxially stretched,xe2x80x9d when used herein to describe a film, means that significant portions of the stretching in each of the two directions are performed simultaneously.
xe2x80x9cstretch ratio,xe2x80x9d as used herein to describe a method of stretching or a stretched film, means the ratio of a linear dimension of a given portion of a stretched film to the linear dimension of the same portion prior to stretching. For example, in a stretched film having an MD stretch ratio (xe2x80x9cMDRxe2x80x9d) of 5:1, a given portion of unstretched film having a 1 cm linear measurement in the machine direction would have 5 cm measurement in the machine direction after stretch. In a stretched film having a TD stretch ratio (xe2x80x9cTDRxe2x80x9d) of 9:1, a given portion of unstretched film having a 1 cm linear measurement in the transverse direction would have 9 cm measurement in the transverse direction after stretch.
xe2x80x9carea stretch ratio,xe2x80x9d means the ratio of the area of a given portion of a stretched film to the area of the same portion prior to stretching. For example, in a biaxially stretched film having an overall area stretch ratio of 50:1, a given 1 cm2 portion of unstretched film would have an area of 50 cm2 after stretching.
Unless context requires otherwise, the terms xe2x80x9corient,xe2x80x9d xe2x80x9cdraw,xe2x80x9d and xe2x80x9cstretchxe2x80x9d are used interchangeably throughout, as are the terms xe2x80x9coriented,xe2x80x9d xe2x80x9cdrawn,xe2x80x9d and xe2x80x9cstretched,xe2x80x9d and the terms xe2x80x9corienting,xe2x80x9d xe2x80x9cdrawing,xe2x80x9d and xe2x80x9cstretching.xe2x80x9d